The invention provides a line output circuit for generating a sawtooth deflection current having a scan period and a flyback period selectively at a first or a second line frequency through a line deflection coil for deflecting an electron beam in a cathode ray display tube where said second frequency is higher than said first frequency, said circuit comprising the series arrangement of a line deflection coil and first or second trace capacitance selected by first switch means which series arrangement is connected across the terminals of a controlled switch, flyback capacitance also being connected across the terminals of said controlled switch, one terminal of the controlled switch being additionally connected to the primary winding of a transformer which winding is additionally arranged to receive a voltage supply form a source of direct voltage, said transformer having a secondary winding one end of which is connected to a rectifier for generating, during the flyback period, the EHT supply for the final anode of a display tube, said controlled switch being conducting during the scan period and being rendered non-conducting at the end of the scan period under the control of line pules at said first or second frequency applied to a control input of said controlled switch to initiate the flyback period.
Such a line output circuit is known from U.S. Pat. No. 4,516,169 and is used in television monitors or data graphic displays for deflecting an electron beam or beams in the line direction across the face of a cathode ray display tube at different line frequencies. The ability to operate at different line frequencies at different times is achieved by changing the supply voltage to the circuit and the switching of the trace capacitance. The requirement to change the supply voltage means that the power supply needs to be of a relatively complex design and makes it difficult to prove an off the shelf design which can readily be adapted to different line frequencies not having a specified relationship. In addition, the circuit of the above patent provides a difference in the ratio between the flyback and line periods for the line frequencies which means that it would be necessary to cause the display to overscan at the lower frequency if the video information is to occupy the same display space at both line frequencies.